Update compatibility build instructions

Author: adamwathan

remark/withPrevalInstructions.js

@@ -201,6 +201,7 @@ function createPrevals({ tool: pageTool = error('UNKNOWN') } = {}) {
         latest: ['tailwindcss@latest', 'postcss@latest', 'autoprefixer@latest'],
         'compat-7': [
           'tailwindcss@npm:@tailwindcss/postcss7-compat',
+          '@tailwindcss/postcss7-compat',
           'postcss@^7',
           'autoprefixer@^9',
         ],

src/pages/docs/installation.mdx

@@ -325,7 +325,7 @@ If you run into the error mentioned above, uninstall Tailwind and re-install usi
 
 ```shell
 npm uninstall tailwindcss postcss autoprefixer
-npm install tailwindcss@npm:@tailwindcss/postcss7-compat postcss@^7 autoprefixer@^9
+npm install tailwindcss@npm:@tailwindcss/postcss7-compat @tailwindcss/postcss7-compat postcss@^7 autoprefixer@^9
 ```
 
 The compatibility build is identical to the main build in every way, so you aren't missing out on any features or anything like that.